Project Summary Macrophages and dendritic cells can acquire small pieces of neighboring cells without killing the donor cell. The acquired material contains a small portion of the donor cell?s cytosol, which can include lipid droplets, functional mitochondria, antigens and even intact intracellular bacteria. There are several intriguing possibilities for how the recipient cells use the acquired material. Our long-term goal is to decipher the role of cytosolic acquisition in the immune response to intracellular bacteria. These discoveries should reveal the impact of a novel, but poorly understood, function of the immune response. Our preliminary data suggests that cytosolic transfer contributes to initiating both the adaptive and innate immune responses as well as enabling certain pathogens to rapidly disseminate. The objective of this proposal is to identify the mechanism of cytosolic acquisition and to define the resulting impact of cytosolic acquisition on priming innate immunity and T cell responses. Our central hypothesis is that cytosolic acquisition by macrophages and dendritic cells enables these cells to acquire immunogenic material from neighboring cells so that they can initiate an innate or adaptive immune response respectively. Our Specific Aims will address this hypothesis in the following ways. Aim 1 will focus on identifying the molecular mechanisms that mediate cytosolic acquisition by antigen presenting cells and to determine the fate of acquired materials. Aim 2 will examine the impact of cytosolic acquisition on bacterial and antigen dissemination, which may improve T cell priming. Aim 3 will define the effect of cytosolic transfer on stimulating host immune responses by recipient macrophages. By completing this project, we will broadly understand the role of cytosolic acquisition on priming the immune response. This contribution is significant because it will establish the general impact of cytosolic acquisition on the immune response and define molecular signals that can be exploited to manipulate cytosolic transfer in more targeted studies. This project is innovative because we are examining a largely undefined immunological process that may be needed to rapidly generate an immune response to intracellular pathogens. If macrophages and dendritic cells are acquiring cytosolic material to initiate an immune response, manipulating this process has the long- term potential to impact the immune priming to a range of diseases, including immune activation against tumors, autoimmunity and Graft versus Host Disease.